On Sunday, Ric Gillespie settled into his living room in central Pennsylvania and flipped on a new History Channel documentary about Amelia Earhart, one that promised “shocking” new evidence that proved what had happened to the famed aviator.

Hundreds of miles away, in Florida, Mike Campbell did the same. Both were armed with a notepad and a healthy amount of skepticism, albeit for different reasons.

For most people, what became of Earhart during her doomed 1937 trip to circumnavigate the globe is a passing, sepia-toned concern, a historic relic occasionally brought up in articles like this one.

Gillespie and Campbell, however, belong to small, competing fraternities of people who have dedicated decades to solving the mystery of Earhart’s disappearance. For them, any new shred of Earhart-related information is enough to make their heart race — or their blood boil — even 80 years after Earhart’s plane vanished.

If there’s one thing the men share, it’s their adamant refusal to believe that Earhart and her navigator, Fred Noonan, crashed into the Pacific Ocean in 1937. The U.S. government accepted this theory and declared Earhart and Noonan dead following a fruitless, months-long search.

Gillespie and the people at the International Group for Historic Aircraft Recovery (TIGHAR) believe Earhart and Noonan were swept off course by strong Pacific winds and crash-landed on what was then called Gardner Island, injured but alive. The pair used the plane’s radio to send distress calls until it was pulled into the ocean. They ultimately died as castaways, Gillespie believes.

Campbell, a retired journalist who authored “Amelia Earhart: The Truth at Last,” insists along with others that Earhart and Noonan were captured in the Marshall Islands by the Japanese, who thought they were American spies, and died in Japanese custody after being tortured.

A photo taken May 20, 1937, shows aviator Amelia Earhart and her Electra plane, taken by Albert Bresnik at Burbank Airport in Burbank, Calif. (Albert Bresnik/The Paragon Agency via AP)

Throughout the decades, the two opposing camps have frequently clashed — gathering evidence, bolstering their own theories and publicly trashing others whenever new Earhart evidence surfaces.

The History Channel documentary sparked the latest conflagration. In previews and media accounts the week before the documentary aired, producers had teased the program with a black-and-white photograph they said had been dredged up from the National Archives.

The grainy picture supposedly showed Earhart and her navigator in Jaluit Harbor in the Marshall Islands after their disappearance — lending credence to the theory that Earhart had survived her final flight and been taken into Japanese custody, according to History Channel representatives.

“When you look at the totality of what we put together and then hold that photograph. … I think that photograph is as close to a smoking gun as you’re going to have in a cold case that’s 80 years old,” Shawn Henry, a researcher featured in the documentary, told The Washington Post last week.

However, after the program aired, a Japanese-military-history blogger matched the photo in question to one first published in a 1935 Japanese travelogue. It couldn’t have depicted Earhart and Noonan, he said, because it had been published at least two years before the pair set off on their trip around the world.

The History Channel released a statement Tuesday acknowledging the questions.

“HISTORY has a team of investigators exploring the latest developments about Amelia Earhart and we will be transparent in our findings,” the statement read. “Ultimately, historical accuracy is most important to us and our viewers.”

In a news release Tuesday, Gillespie said he suspected the photo wasn’t really of Earhart and Noonan even before he sat down on his couch and turned to the History Channel.

He outlined a litany of reasons: “The picture was undated; there were no Japanese, no guards, the figure alleged to be Amelia had hair that was much too long; the identification of a man as Fred Noonan depended upon a photo of Noonan that had been reversed to make the hairline match; their clothes were wrong; an indistinct blob behind a ship was proclaimed to be Earhart’s Electra on a barge — and yet, to dozens of media outlets, the photo was heralded as ‘proof’ of Earhart’s fate.”

Gillespie dismissed the History Channel documentary as a rehash of “thoroughly debunked Japanese capture nonsense.” He also noted that his wife had refused to watch the show with him.

“She’s just not going to deal with me in that situation,” he said. “I’ve got my notepad in front of me. And I’m laughing out loud to myself, talking to the screen. ‘You didn’t just say that!’ ”

Meanwhile, in Florida, Campbell finished the documentary with mixed feelings. On his blog, he later blasted the History Channel for its “bogus photo claims” — marking a rare instance in which he and Gillespie agreed.

On the other hand, Campbell told The Post that it had been a “pleasant surprise” to see all the eyewitness accounts in the documentary, including firsthand and secondhand interviews from the Marshall Islands and Saipan. Those included “still mentally sharp” 91-year-old Josephine Blanco Akiyama, he said, who told investigators she saw Earhart being taken into Japanese custody in Saipan, as well as old footage of other eyewitnesses talking about Noonan being treated aboard the “Koshu” Japanese merchant ship as Amelia watched.

“These witnesses are magnificent, significant and revealing figures whose convincing accounts, if known and accepted by enough concerned Americans, might help unlock the deepest locks in Washington, the ones with the top-secret Earhart files,” Campbell told The Post.

It was a shame that the hubbub over the grainy photo had undermined what had been perfectly good evidence presented in the documentary, Campbell said.

“The evidence presented in the documentary is all old evidence that researchers like me know about,” Campbell said. “To use the photo as a predicate to enter into this History Channel kind of a bogus investigation thing is just ridiculous. They did put some very good information into that program that hasn’t been seen by the public.”

In his book, Campbell accuses the U.S. government of covering up what he says really happened to Earhart and Noonan: that they were captured by the Japanese, taken to Saipan and killed. He cites a 1960 Office of Naval Intelligence report as proof of “the government’s ongoing interest in the Earhart case — an activity that continues to this day, quietly and behind the scenes.”

Gillespie continues to look for evidence to support his Earhart theory. His group recently went on a National Geographic-sponsored expedition with TIGHAR to Nikumaroro, or Gardner Island, hoping to find the spot where Earhart died. Their secret weapon: border collies trained to sniff out human remains. TIGHAR hoped that the dogs would help locate a bone that could provide a DNA link to Earhart. It’s one of a dozen TIGHAR expeditions that have not produced a smoking gun.

When asked about TIGHAR’s latest expeditions, Campbell refused to comment.

“I don’t have anything to do with Gillespie or TIGHAR … I don’t want to attack him here. It’s not relevant anymore,” he said. “This has been such a cottage industry for these frauds and I’m sitting back here with 5,000 threads, a book that would kill a horse with information that absolutely proves she was there (on the Marshall Islands).”

The men who lead such expeditions — and each school of Earhart theory — have strikingly similar views of their universe: They each think they know the truth about what happened to Amelia Earhart and are one piece of evidence away from proving it to the world. And they wish other folks would stop spouting bogus theories in public.

“This is like fundamentalist ministries of the Deep South,” Gillespie said. “This guy’s got the ministry of the laying on of hands. And this guy’s got the snakes. And this guy’s got the speaking in tongues. It gets really passionate and everybody ends up screaming at each other.”